1. Technical Field
This invention relates to an oxysulfide thin film having a light emitting function, and more particularly, to a phosphor thin film used as a light-emitting layer in inorganic EL devices and an EL panel using the same.
2. Background Art
In the recent years, active research works have been made on thin-film EL devices as small-size and large-size, lightweight flat displays. A monochromatic thin-film EL display using a phosphor thin film of manganese-doped zinc sulfide capable of emitting yellowish orange light has already become commercially practical as a dual insulated structure using thin-film insulating layers 2 and 4 as shown in FIG. 2. In FIG. 2, a predetermined pattern of lower electrodes 5 is formed on a substrate 1, and a first insulating layer 2 is formed on the lower electrodes 5. On the first insulating layer 2, a light-emitting layer 3 and a second insulating layer 4 are successively formed. On the second insulating layer 4, a predetermined pattern of upper electrodes 6 is formed so as to construct a matrix circuit with the lower electrodes 5.
Thin-film EL displays must display images in color in order that they find use as computer, TV and similar monitors. Thin-film EL displays using sulfide phosphor thin films are fully reliable and resistant to environment, but at present regarded unsuitable as color displays because EL phosphors required to emit light in the primaries of red, green and blue have poor characteristics. Engineers continued research on SrS:Ce (using SrS as a matrix material and Ce as a luminescent center) and ZnS:Tm as a candidate for the blue light-emitting phosphor, ZnS:Sm and CaS:Eu as a candidate for the red light-emitting phosphor, and ZnS:Tb and CaS:Ce as a candidate for the green light-emitting phosphor.
These phosphor thin films capable of emitting light in the primaries of red, green and blue suffer from problems of emission luminance, emission efficiency and color purity. Thus color EL panels have not reached the commercial stage. Referring to the blue color among others, a relatively high luminance is achieved using SrS:Ce. However, its luminance is still short as the blue color for full-color display and its chromaticity is shifted toward green. There is a desire to have a better blue light-emitting layer.
To solve the above problem, thiogallate and thioaluminate base blue phosphors such as SrGa2S4:Ce, CaGa2S4:Ce, and BaAl2S4:Eu were developed as described in JP-A 7-122364, JP-A 8-134440, Shinshu Univ. Technical Report, EID 98-113, pp. 19-24, and Jpn. J. Appl. Phys., Vol. 38 (1999), pp. L1291-1292. These thiogallate base phosphors are satisfactory in color purity, but suffer from a low luminance and especially, difficulty to form a thin film of uniform composition because of the multi-component composition. It is believed that thin films of quality are not obtainable because of poor crystallinity resulting from inconvenient composition control, formation of defects resulting from sulfur removal, and admittance of impurities; and these factors lead to a failure to increase the luminance. In particular, thioaluminate base phosphors are quite difficult to control their composition.
Contemplated for the formation of thioaluminate base thin films are a method of forming a target having the same composition as the end BaAl2S4:Eu thin film to be formed and sputtering the target to form a light emitting layer as described in JP-A 8-134440; and a method of forming two pellets of BaS:Eu and Al2S3, and evaporating them by a two-source pulse electron beam evaporation process, thereby depositing a BaAl2S4:Eu thin film as described in Jpn. J. Appl. Phys., Vol. 38 (1999), pp. L1291-1292.
JP-A 7-122364 discloses a method for forming a SrIn2S4:Eu light emitting layer by evaporating Sr metal, In metal and EuCl3 by a molecular beam epitaxy (MBE) process in a vacuum chamber into which H2S gas is admitted, thereby depositing a SrIn2S4:Eu light emitting layer on a substrate. This method, however, is quite difficult to control the sources of metals for the matrix material (SrIn2S4) and the source of luminescent center substance (Eu) so as to accurately control the quantity of the luminescent center. For example, it is almost impossible with the current evaporation process that the molar ratio of Sr to In is controlled to 1:1 to induce sulfidation reaction with H2S, the molar ratio of the matrix material to Eu is controlled to 99.5:0.1, and the variation of that 0.1 unit quantity of Ce is within 5%. Meanwhile, in the case of Al electrodes used as the electrodes of LSI, the variation in thickness of Al thin film is about 5% even when the evaporation source is relatively stable. It is then understood that controlling the concentration of Eu to a precision within 5% is very difficult.
As to red and green EL thin films aside from the blue EL thin film, for example, red light emitting phosphors ZnS:Sm and CaS:Eu and green light emitting phosphors ZnS:Tb and CaS:Ce, phosphor thin films capable of emitting light at a relatively high luminance can be produced by forming targets or pellets of the desired composition and vapor depositing them by a sputtering or EB evaporation process.
In order to develop practical full-color EL panels, phosphor materials capable of providing blue, green and red phosphors in a consistent manner and at a low cost and methods of preparing such phosphors are necessary. Since matrix materials of phosphor thin films and luminescent center materials individually have differing chemical or physical properties as described above, the preparation method differs depending on the identity of the phosphor thin film. A film forming method capable of deriving a high luminance from one material fails to achieve a high luminance for a phosphor thin film of another color. For the overall process of manufacturing a full-color EL panel, plural types of film forming apparatus are necessary. The manufacturing process becomes very complex, leading to an increased cost of panel manufacture.
Moreover, the electroluminescent spectra of the aforementioned blue, green and red EL phosphor thin films are all broad. When they are used in a full-color EL panel, RGB necessary as the panel must be cut out of the EL spectra of the EL phosphor thin films using filters. Use of filters complicates the manufacture process and, still worse, brings about a lowering of luminance. When RGB is taken out through filters, the luminance of blue, green and red EL phosphor thin films is lost by 10 to 50% so that the luminance is reduced below the practically acceptable level.
To solve the above-discussed problem, there remains a need for red, green and blue phosphor thin film materials capable of emitting light of a satisfactory color purity without a need for filters and at a high luminance, as well as phosphor matrix materials and luminescent center materials of similar chemical or physical properties which can be prepared to a high luminance capability by an identical film-forming method or film-forming apparatus.
An object of the invention is to provide a phosphor thin film eliminating a need for filters, having a satisfactory color purity and best suited as RGB in full-color EL displays, a method for preparing the same, and an EL panel.
Another object is to provide a phosphor thin film which enables to simplify the manufacture process of full-color EL panels, and offers the advantages of minimized variation of luminance, increased yields, and reduced manufacture cost, a method for preparing the same, and an EL panel.
These and other objects are attained by the present invention which is defined below as (1) to (8).
(1) A phosphor thin film formed of a matrix material comprising an oxysulfide consisting of at least one compound selected from rare earth thioaluminates, rare earth thiogallates and rare earth thioindates, in which oxygen is incorporated, said matrix material further containing an element serving as a luminescent center.
said matrix material further containing an element serving as a luminescent center.
(2) The phosphor thin film of (1) wherein the molar ratio of oxygen element to sulfur element in said oxysulfide, as expressed by O/(S+O), is in the range:
O/(S+O)=0.01 to 0.85.
(3) The phosphor thin film of (1) or (2) having the following compositional formula:
RxAyOzSw:M 
wherein M is a metal element serving as the luminescent center, R is at least one element selected from rare earth elements, A is at least one element selected from Al, Ga and In, x is in the range of 1 to 5, y is in the range of 1 to 15, z is in the range of 3 to 30, and w is in the range of 3 to 30.
(4) The phosphor thin film of any one of (1) to (3) wherein said luminescent center is provided by a rare earth element.
(5) An EL panel having the phosphor thin film of any one of (1) to (4).
(6) A method for preparing the phosphor thin film of any one of (1) to (4), comprising
forming a sulfide thin film, and
annealing the thin film in an oxidizing atmosphere.
(7) A method for preparing the phosphor thin film of any one of (1) to (4) by an evaporation process, comprising
placing at least one evaporation source selected from aluminum sulfide, gallium sulfide and indium sulfide and an evaporation source of a rare earth sulfide having a luminescent center added thereto in a vacuum chamber, admitting oxygen gas into the vacuum chamber, and
evaporating at least one compound selected from aluminum sulfide, gallium sulfide and indium sulfide and the rare earth sulfide substance from the respective sources and depositing the evaporated substances on a substrate while binding the substances with oxygen gas, thereby forming a phosphor thin film.
(8) A method for preparing the phosphor thin film of (1) by an evaporation process, comprising
placing at least one evaporation source selected from aluminum sulfide, gallium sulfide and indium sulfide and an evaporation source of an rare earth metal or an rare earth sulfide having a luminescent center added thereto in a vacuum chamber, admitting hydrogen sulfide gas into the vacuum chamber,
evaporating at least one compound selected from aluminum sulfide, gallium sulfide and indium sulfide and the rare earth sulfide substance or rare earth metal substance from the respective sources and depositing the evaporated substances on a substrate while binding the substances with hydrogen sulfide gas, thereby forming a sulfide phosphor thin film, and
annealing the thin film in an oxidizing atmosphere.
The present invention is arrived at by synthesizing compound materials, using a reactive deposition process as the common film forming process and chemically or physically stable oxides. The resulting phosphor thin films are able to emit light of different color covering a wide spectrum from red to blue.
The inventors formed thin films of rare earth thioaluminates, rare earth thiogallates and rare earth thioindates as thin film phosphors intended for EL application. EL devices were prepared using the thin films, but they failed to produce the desired light emission. The thin films had an emission luminance as low as 2 cd/m2 at the highest. The luminance must be increased in order that the thin films be applied to EL device panels.
Based on these empirical results, the inventors continued research on phosphor thin films of the above series and reached the present invention. It has been found that an outstanding increase of luminance is accomplished by adding oxygen to rare earth thioaluminate, rare earth thiogallate and rare earth thioindate matrix materials to form oxysulfides.